The present invention relates to the art of nanotechnology, and in particular, to carbon nanotube technology, its function and structure.
A carbon nanotube is a single graphene sheet in the form of a seamless cylinder. The ends of a nanotube typically have hemispherical caps. The typical diameter of a nanotube ranges from about 1 nm to 10 nm. The length of a nanotube potentially can be millions of times greater than its diameter.
Carbon nanotubes are comprised of shells of sp2-hybridized carbon atoms forming a hexagonal network that is itself arranged helically within the cylinder. Basically, helicity is the arrangement of the carbon hexagonal rings with respect to a defined axis of a tube. (M. S. Dresselhaus et al “Science of Fullerenes and Carbon Nanotubes” (Academic Press, New York, 1996)).
Carbon nanotubes are grown by combining a source of carbon with a catalytic nanostructured material such as iron or cobalt at elevated temperatures. At such temperatures, the catalyst has a high solubility for carbon. The carbon links up to form graphene and wraps around the catalyst to form a cylinder. Subsequent growth occurs from the further addition of carbon.
Since their discovery in the early 1990s, carbon nanotubes have been the focus of intense study due to their very desirable and unique combination of physical properties. They are chemically inert, thermally stable, highly strong, lightweight, flexible and electrically conductive. In fact, carbon nanotubes may potentially be stiffer and stronger than any other known material.
Carbon nanotubes are currently being proposed for numerous applications, such as, for example, catalyst supports in heterogeneous catalysis, high strength engineering fibers, sensory devices and molecular wires for the next generation of electronics devices.
There has been particularly intense study of the electrical properties of nanotubes, and their potential applications in electronics. Metallic carbon nanotubes have conductivities and current densities that meet or exceed the best metals; and semiconducting carbon nanotubes have mobilities and transconductance that meet or exceed the best semiconductors.
The physical properties of carbon nanotubes are structure-dependent. For example, depending on the diameter and helicity of a nanotube, the tube can be either metallic or semi-conducting. Also, a single structural defect in a hexagonal ring can change a metallic nanotube to a semiconducting nanotube.
Current methods for producing nanotubes do not allow for the control of the structural properties of nanotubes in a reliable, rapid and reproducible manner. A mixture of tubes with diverse diameters, helicities and structural defects are produced. Thus, a mixture of metallic and semi-conductng nanotubes are produced. This presents a major obstacle to actualizing the utility of carbon nanotubes for end use applications.
Accordingly, there remains a need for a method of producing carbon nanotubes with the particular desired physical properties which are necessary for various end use applications.